Carl and Jerry: The Crazy Clock CaperOctober 1960 Popular Electronics

Wax nostalgic about and learn from the history of early electronics. See articles
from Popular Electronics,
published October 1954 - April 1985. All copyrights are hereby acknowledged.

Before there were clocks that synchronized themselves to a wireless low frequency (LF) time standard emanating from one of NIST's broadcast towers, a different method was used to keep all the clocks in a building (like a school) reading the same time. Many of the AC-powered mechanical master-slave clock systems are still in use today. This episode of Carl and Jerry has them teaming with a contract repairman to figure out why seemingly random clocks in their high school failed to synch with the master overnight. Author John T. Frye provides a pretty thorough overview of how the system operates using a power line carrier scheme. Of course the boys' keen troubleshooting skills come to the rescue of the poor, beleaguered repairman. I still remember watching in awe one time in elementary school (c1960s) when after the end of a power failure during the school day the wall clock at the front of the room began racing ahead seemingly magically until it stopped at what was presumably the correct time. If I was the teacher, I would have told the kids that it was the hand of God adjusting the clock, and that He was watching them to administer swift punishment if anyone got out of line - that's why I'm not a teacher ;-)

Carl and Jerry: The Crazy Clock Caper

By John T. Frye W9EGV

It was almost the end of an exasperatingly beautiful, warm, sunny school day. Carl and Jerry were sitting in class listening impatiently to the voice of the Latin teacher droning on and on about the second conjugation. Their eyes were on the clock over the door.

Suddenly the voice of the principal issued from the intercom speaker: "Miss Manders, will you please have Jerry Bishop and Carl Anderson come to my office at once?"

The boys rose from their seats at a nod from Miss Manders and started for the door. They could feel the backs of their necks growing hot under the concentrated questioning stares of their classmates.

"Now what've we done?" Carl muttered as they walked along the hall.

"Rather: what have they caught us at?"

Jerry whispered.

Jerry knocked at the office door, and the sight of the principal's smiling face banished their worries.

"Boys, this is Mr. Stoner from Center City," he said. "Mr. Stoner is here to straighten out a little trouble we're having with our new automatic clock and bell system."

"He needs a couple of boys to help him with his testing," the principal explained. "I suggested you two because of your interest in electricity and electronics. I have to leave for a board meeting, but I'm sure you three can get along without my help - especially since my wife says I can't even plug in the electric toaster and do it right!"

As the principal closed the door behind him, Mr. Stoner slumped into a chair. Nervously tugging at his ear, he stared searchingly into the faces of the two boys. Finally he spoke:

"Boys, I'm going to level with you. I'm in a spot. Actually, I'm an electric typewriter serviceman. The man who is supposed to take care of these clocks is on vacation, and I'm pinch-hitting for him. I know just a little about the system, but that little doesn't seem to be enough to find the trouble. I've spent three days on it, and my boss is beginning to ride me. He thinks a man who can fix electric typewriters should be able to fix anything. On top of that, my wife called last night and said that my little boy is sick - I should be home with them.

"The principal tells me you two are sharp on electronics. I hope he's right, for I certainly could use some help."

"What's wrong?" Jerry asked.

"All the clocks in the building are supposed to keep in step with the master clock here in the office," Mr. Stoner replied, as he sprang up and renewed his pacing. "Every fifty-ninth minute this master clock causes an audio tone of a certain frequency to be fed into a power amplifier located there in the closet. The signal is built up to about forty watts and fed into the 117-volt a.c. line. It goes out over the power lines to the electric clocks plugged in in the various rooms.

"Inside each clock is a transformer with tuned windings. The primary in series with a capacitor is connected directly across the a.c. line. The coil and capacitor are series-resonant at the audio frequency, so maximum current flows in the primary. Audio voltage developed across the parallel-tuned secondary fires a cold-cathode thyratron tube. Current through this thyratron actuates an electric clutch that causes the sweeping second hand to pick up the minute hand and carry it to the vertical position before dropping it. Every twelve hours a similar arrangement corrects the hour hand.

"In some installations the correction takes place at 6 a.m. and 6 p.m., but the hour hand is corrected at noon and midnight in this setup. Different audio frequencies are fed into the line by the clock at preset times. These signals are picked up by other tuned transformers with thyratrons that close relays and ring bells in the classrooms. By using different frequencies, the bells of different rooms can be rung at different times so a complex time schedule can be accommodated.

"Every morning several of the room clocks indicate the wrong hour. Others are on time. Different clocks are incorrect on different mornings. At noon they are all automatically corrected, and they stay on time until school is out. But the next morning it's the same old story."

"What have you done so far?" Carl wanted to know.

"I've checked the tone generator and the power amplifier thoroughly. All the tones are on frequency, and there's no parasitic oscillation or noise in the amplifier. I've checked the tuning of the transformers in the clocks to make sure they're right on frequency. I've gone over the wiring. And I've measured the clock-setting signal at all the clocks - it's supposed to be in excess of 0.8 volt, and it is.

"Incidentally, the coupling between the primary and secondary of each tuned transformer is variable so that the voltage delivered to the thyratrons can be kept uniform in spite of different audio voltage levels present across the wall sockets into which the clocks are plugged. Since the audio signal must thread its way through the maze of a.c. wiring and be subjected to various bypassing actions of different loads on different parts of the wiring, it is understandable that these levels would be different."

"It's kind of funny that nothing happens during the day," Jerry mused. "Maybe something the janitors do at night upsets the clocks."

"I thought of that. The only electrical apparatus they use regularly is a power vacuum sweeper. When the vacuum is running, it produces some noise on the line, but this noise only measures 0.2 volt - far too low to trip the clock-setting mechanism. Oh, yes, there's one odd thing the principal noticed: more clocks seem to go crazy when it rains. That would point toward humidity as the cause of our trouble, but I can't imagine how."

"Do you have any other ideas?" Carl wanted to know.

"Just one. Today I figured that line voltage variation occurring at night might somehow upset things, so I borrowed this variable-voltage transformer from a TV shop. I'll crank the voltage applied to the signal generator and the power amplifier up and down while you two check the clocks in various rooms to see if anything happens."

Carl and Jerry went from room to room inspecting the clocks while Mr. Stoner raised and then lowered the line voltage applied to the clock-regulating equipment by ten percent. The clocks never budged.

"Well, there goes my last idea," Mr. Stoner said dispiritedly as the boys came back into the office. "I just don't know -"

He was interrupted by the ringing of the telephone on the desk. He answered it, and the boys could see him becoming more and more agitated as he talked.

"My little boy has just been taken to the hospital for an emergency appendectomy," he reported as he hung up the telephone. He began gathering up his tools and throwing them into his tool box. I must go home at once. Have the janitor lock the office. I don't know when I'll get back." This last sentence was shouted back over his shoulder as he dashed out the office door.

Carl and Jerry hunted up the janitor and delivered the message, then started for home.

"You know," Carl remarked, "I feel sorry for Mr. Stoner. He really has trouble. I wish we could help him."

"Maybe we can," Jerry answered. "Let's go to school a half hour early tomorrow and check those clocks ourselves. We just might get lucky and stumble onto something."

The next morning the boys found only seven of the forty-one clocks with the incorrect time. Two were in the basement, three on the first floor, and two on the top floor.

Shortly before noon it began to rain, so Carl and Jerry ate lunch in the school cafeteria. As they ate, they puzzled over their problem. "It simply has to be something that happens between midnight and morning," Jerry finally decided. "Suppose we ask the principal to let us snoop around here tonight and see what goes on."

The principal readily agreed to the plan and gave the boys a pass key that would let them into any of the classrooms. The sensible thing would have been for the boys to go to bed right after supper and get some sleep before midnight, but what did our heroes do? They stayed up and watched the late show until a quarter of twelve! Then they set out for the school, Jerry carrying an a.c. voltmeter, and Carl a pair of binoculars.

Quietly, they let themselves into the dimly-lit building. There was something spooky and a little sinister about the empty halls and the closed doors of the classrooms. From somewhere in the building came a faint humming sound, and they moved softly about in their sneakers until they located it. The hum was coming from the large tank of an industrial vacuum sweeper which was sitting on a low cart in front of an open door.

As the boys peered around a corner, the janitor came out of the room and piled the long flexible hose of the vacuum cleaner on the cart, recovered the line cord that had been plugged into a socket in the room, and pushed the cart to the next door.

It took the janitor only a few minutes to vacuum this room, but he apparently decided that the floor of the next one was too dirty to be dry-cleaned. First he sloshed sudsy water over the floor and gave it a quick going-over with a rotary wet mop. Then he used the vacuum to suck up the excess water. As he did this, the boys could hear the motor of the cleaner slowing down in protest. Finally, he went over the floor with a clean mop and clear water.

Jerry silently beckoned Carl into a class­room across the hall. The floor was still damp, and the clock was four hours fast!

"I've got an idea," Jerry whispered. "You take the key and get into that wing across the way where you can see the clocks in the rooms the janitor is cleaning. Don't let him see you. I'll be doing some checking here. We'll meet in this room in half an hour."

Carl waited until the janitor had started on another room and then slipped away.

Jerry tiptoed across the hall to a baseboard outlet socket just outside the room in which the janitor was working. There he plugged in his own version of a tuned transformer he had made from an old TV flyback transformer. Placing his voltmeter across the secondary of this transformer enabled him to read the voltage of any clock-setting signal on the line without interference from the 60-cycle a.c. current.

When the janitor switched on the vacuum cleaner, Jerry got a reading that represented 0.2 volt; but when the sweeper began to suck up water, this reading quadrupled! With a smile of satisfaction, Jerry unplugged his apparatus and returned to the rendezvous room to await Carl.

The latter soon appeared, his eyes wide with excitement, and his uncased binoculars dangling about his neck. "When that vacuum sweeper begins to suck up water, the clock in the room goes crazy," Carl reported.

They did not discuss the matter further. It was two o'clock, and both boys were growing very sleepy. They slipped out of the building and went straight home to bed.

When Carl and Jerry arrived at school the next morning, Mr. Stoner was standing on the steps, smiling and relaxed.

"The little boy is getting alone fine," he said, "and I feel like a new man. I guess I needed a shock to show me what was really important. Now that my son is going to be all right, nothing else bothers me. We'll lick this clock thing in time, and I refuse to get worked up about it again."

Excitedly, both boys talking at once, Carl and Jerry told him what they had observed the previous night.

"That's it!" Mr. Stoner exclaimed. "The vacuum cleaner was only sucking air when I checked it. When it sucks water, the motor works harder and produces a noise of the right amplitude and frequency to trip the clock-setting mechanism in the room where the vacuum sweeper is being used. It's too weak to bother more distant clocks, and even the clock in the room isn't disturbed during the dry-cleaning process. Well, installing a noise filter inside the motor housing will stop the noise in a jiffy."

"Only one thing bothers me," Carl said slowly. "Why did more clocks get out of synch when it rained?"

"That's easy," Jerry broke in. "When it rained, the kids tracked in more mud, making it necessary to scrub more rooms that night. So more clocks were off the next morning."

"I don't know how to thank you fellows," Mr. Stoner said sincerely.

"Well," Carl suggested slyly, "you could say you needed us to help you install that noise filter during the Latin period. We're due to have a quiz, and -"

"Say no more!" Mr. Stoner interrupted with a chuckle. "I'm sure I need you more than Caesar does today."

Posted July 4, 2019 (original 11/4/2014)

Carl & Jerry: Their Complete Adventures is now available. "From
1954 through 1964, Popular Electronics published 119 adventures of Carl Anderson and Jerry Bishop, two teen boys with
a passion for electronics and a knack for getting into and out of trouble with haywire lash-ups built
in Jerry's basement. Better still, the boys explained how it all worked, and in doing so, launched countless
young people into careers in science and technology. Now, for the first time ever, the full run of Carl
and Jerry yarns by John T. Frye are available again, in five authorized anthologies that include the
full text and all illustrations."

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